Primary acinic cell carcinoma of the trachea: A case report and literature review

Rationale: Salivary gland-type acinic cell carcinoma (ACC) is a low-grade malignancy. Primary ACC of the trachea and lungs is rare; here, we describe 1 such case. The histological morphology of tracheal ACC was similar to that of its salivary gland-associated equivalent. Because of its rarity, it is easily misdiagnosed as another type of tracheal or lung tumor. Microscopic analysis of pathological features and immunohistochemistry help diagnose primary ACC of the trachea and lungs. Patient concerns: A 33-year-old female complained of shortness of breath and hemoptysis for 2 years, and reported the symptoms to have aggravated over the last 4 months. The patient was admitted to our hospital for further treatment. Enhanced computed tomography revealed a soft tissue density nodule shadow in the trachea, which was approximately 1.3 × 1.2 cm in size. Diagnoses: Based on the clinical information, morphological features, and immunohistochemistry, the pathological diagnosis was primary ACC of the trachea. Intervention: The tracheal lesion was resected with an electric snare, electrotomy, freezing, and an argon knife using a rigid bronchoscope. Outcomes: The patient’s postoperative course was uneventful. Lessons: It is important to prevent misdiagnosis of this type of tumor as another type of lung tumor. Morphological and immunohistochemical features can be useful in diagnosing primary ACC of the trachea and lungs.


Introduction
Salivary gland-type acinic cell carcinoma (ACC) is a relatively low grade malignancy. [1] It usually occurs in the salivary glands of the head and neck region, particularly in the parotid gland. Apart from the salivary gland, ACC can also occur in the trachea, lungs, mammary gland, and other organs among which primary ACC of the trachea and lung is very rare. [1][2][3] 2. Case presentation

Ethical approval
This study was approved by the Institutional Review Board of China Medical University for Human Studies. The ethical board approval number is LS[2021]009. Written informed consent was obtained from the patient for publication of this case report and accompanying images. This study was conducted in accordance with the principles of the Declaration of Helsinki.

Clinical history
A 33-year-old female complained of shortness of breath, coughing, purulent yellow sputum, and bloody sputum 2 years prior to presentation. Four months prior to presentation, the symptoms worsened, including shortness of breath, fatigue, coughing, excessive yellow sputum production, and massive hemoptysis. The patient had no fever, chest pain, or other relevant symptoms. The patient was admitted to another hospital 6 days prior to presentation and received pharmaceuticals for symptomatic treatment, such as "cefodizime," "ipratropium bromide," and "N-acetylcysteine." The patient's symptoms were not significantly relieved. She was admitted to our hospital for further treatment. Enhanced computed tomography revealed a soft tissue density nodule shadow in the tracheal cavity. The size of the lesion was about 1.3 × 1.2 cm with a computed tomography value of about 57 HU. The tracheal lumen was narrow and the tracheal ring was complete. The remaining bronchi were unobstructed, without expansion or stenosis. The tracheobronchial and subcarinal lymph nodes were not enlarged (Fig. 1). The preoperative diagnosis was intratracheal mass. Resection of the tracheal lesion was then performed using a rigid bronchoscope. During surgery, a tumor mass was observed to protrude into the lumen of the trachea, 6 cm below the glottis, and the lumen was narrow. The tumor was rough and uneven on the surface, with abundant blood supply. The tumor was completely removed with an electric snare, electrotomy, freezing, and an argon knife. The patient did not receive postoperative radiotherapy or chemotherapy and recovered well after surgery. Follow-up at 13 months showed no evidence of recurrence or other metastatic diseases.

Morphological and immunohistochemical findings
Microscopically, the tumor tissue was located under the bronchial mucosa and exhibited no morphological transition to the bronchial epithelia. The tumor cells formed solid cell nests and acinar or microcystic structures (Fig. 2). There was very little fibrous stroma, and the blood vessels were thin and abundant. The tumor cells were relatively uniform, round or polygonal in shape, with abundant granular eosinophilic or clear cytoplasm. Mucinous secretions have also been observed for some tumor cells. The cell nuclei were small, uniform, and bland with fine chromatin. Nucleoli and mitotic figures were not observed. There were no signs of lymphatic, vascular, or neural invasion.

Discussion
Based on the clinical information, morphological features, and immunohistochemistry results described above, the tumor was diagnosed as primary ACC of the trachea. Primary ACC of the trachea and lungs usually presents as an endobronchial mass because it originates from the submucosal glands. Primary salivary gland-type tumors of the lungs are rare, constituting < 1% of all pulmonary tumors. Primary lung ACC is a low-grade malignant tumor that usually presents as an isolated tumor adjacent to or close to the bronchus. [1,4] In 1972, Fechner et al first reported a case of primary ACC of the lung. [5] To the best of our knowledge, including the present case, only 32 patients with primary ACC of the lung or trachea have been reported so far.  Clinicopathological features of the patients are summarized in Table 1. The most common symptoms were hemoptysis and coughing, but 1-third (11/32) of the patients were asymptomatic and the tumor was identified incidentally.  Of the patients with primary ACC of the lungs, 14 were female and 17 were male. The patient age varied widely between 4 to 75 years (mean age: 43.4 years). The diameter of the tumors ranged from 0.4 to 8.6 cm (mean diameter: 2.6 cm). Twenty cases occurred in the right lung, 7 in the left lung, and 5 in the trachea. Two cases of lymph node metastasis have been reported. [12,13] The histological features of this type of tumor are extremely similar to those of ACC of the salivary glands in the head and neck region. Primary ACC of the lung or trachea may be confused with other primary or metastatic lung tumors.  be identified based on clinical history. The presence of PAX8 and CD10 markers can help identify its renal origin. Carcinoid tumors with clear or eosinophilic cell components usually stain positive for chromogranin A, synaptophysin, and CD56, while ACC stains negative for these markers. Mammary analogue secretory carcinomas can also occur in the lungs and are very similar to ACC in morphology, making it difficult to differentiate between the 2 types. [27,28] However, mammary analogue  secretory carcinoma of the lung was also observed to harbor ETV6-NTRK3 rearrangements and was positive for mammaglobin and S-100, which cannot be detected in cases of ACC. [27] Because ACC of the lung is rare, its definitive histopathological prognostic factors have not been elucidated. All 32 patients reported underwent surgery. Of those with documented follow-up, only 1 patient had tumor recurrence 20 months after surgery. Thus, ACC of the lung or trachea is classified as a lowgrade tumor. Complete surgical resection of ACC of the lung or trachea is curative and feasible.